A steel sheet for tinplate and a TFS steel sheet is generally manufactured selectively in accordance with a Rockwell hardness by once conducting temper rolling depending on a temper grade of tinplate to produce products having hardness of T1 to T5 (target Rockwell hardness (HR30T) of 49±3 to 65±3) and then conducting double reduce rolling to produce products having a temper grade of DR6 to DR10 (target Rockwell hardness (HR30T) of 78±3 to 80±3). To manufacture hard tinplate, therefore, a C content is increased so as to achieve a predetermined hardness range of the temper grade. Even when the hardness remains within the predetermined range, however, products fail, in some cases, to provide formability when the formability is required simultaneously with the hardness. When ultra-low C content is adopted to satisfy the formability, the formability can be satisfied but the hardness undesirably drops below a predetermined value.
To improve formability, a method for manufacturing a steel sheet excellent in both formability and corrosion resistance by reducing the C content to a ultra-low level, adding solid solution hardening elements such as Nb and B, and optimizing a soaking time and a cooling time of an annealing condition is described in Japanese Unexamined Patent Publication (Kokai) No. 9-157757, and so forth.
However, because the ASTM standard stipulates the upper limit amount of the solid solution hardening elements for the steel sheet for tinplate, sufficient amounts of addition elements cannot be added and hardening of the steel sheets becomes insufficient. Furthermore, the addition of Nb, B, etc, raises the annealing re-crystallization temperature and results in deterioration of a passing property an causes drifting of sheet and shape defects during annealing.
It has therefore been desired to develop a manufacturing method of a steel sheet capable of securing formability without adding solid solution hardening elements, if possible, and acquiring hardness of a required level.
Steel sheets such as tinplate and tin-free steel (TFS) sheets have gained a wide application as materials for plate working to produce cans. Production methods of two-piece cans as one of the forms of the cans include the following two systems. The first is a DI (Drawing and Ironing) system that once punches out the material into a metal cap shape, thinly draws a wall surface of a can and applies so-called “ironing”. Printing is thereafter applied to the outer surface of the can. The material for this DI can has a relatively large thickness of 0.24 to 0.3 mm because ironing is applied.
The other system is DRD (Drawing and Re-drawing). The material is once punched out into a metal cap shape and is again punched out and shaped. Printing is thereafter applied to the outer surface of the can. Because ironing of the DRD cans is not necessary, the material for the DRD cans is as thin as 0.2 mm or below. Because the material for the DRD cans is thin and must satisfy the predetermined compression strength, a high strength is required for the material. To secure the formability of the cans, the material must also have a small anisotropy.
Japanese Unexamined Patent Publication (Kokai) No. 61-69928 describes a manufacturing method of a steel sheet for DI processing having a temper grade of T2 to T4 which method reduces a C content to a ultra-low level and an O content to a low level to improve the formability, and stipulates a hot rolling condition, a cold rolling condition and a continuous annealing condition. However, the method of this patent document deals with steel sheets having a thickness of more than 0.2 mm but is not directed to steel sheets having a thickness of 0.2 mm or below.
Nonetheless, materials having low anisotropy are required in many cases for these can materials in order to prevent variance of deformation that occurs during processing after punch-out and printing to the surface, and to suppress the occurrence of a non-uniform ear or a so-called “earing” at the mouth edge of the can that occurs during deep drawing and causes a problem in cap processing.
Distortion printing materials, to which deep drawing is applied after printing is done on a steel sheet, are known in Europe, for example. In this case, punching and printing of the steel sheets are carried out while anisotropy of the steel sheets is taken into consideration.
When the earing is great, variance of anisotropy itself becomes great, too, and deviation of printing occurs. Therefore, users require a Δr value of ≦approx. 0.2.
Cap processing includes trimming to align the ear portion but an attempt has been made to omit this trimming step. To eliminate the trimming step, the earing must be made small and users have required a further decrease the Δr value representing anisotropy to Δr value≦approx. 0.1. This Δr value will be described later.
Incidentally, an r value is known as one of the indices for evaluating the deformation characteristics of materials in deep drawing. This r value is called a “plastic strain ratio” and is defined by the following formula
                    r        =                ⁢                              ln            ⁡                          (                                                W                  o                                /                W                            )                                /                      ln            ⁡                          (                                                T                  o                                /                T                            )                                                              =                ⁢                              ln            ⁡                          (                                                W                  o                                /                W                            )                                /                      ln            ⁡                          (                                                LW                  /                                      L                    o                                                  ⁢                                  W                  o                                            )                                          when Lo, Wo and To as a first gauge length of a tensile test piece, a width of a parallel portion and a thickness, respectively, change to L, W, T after tensile deformation within a range in which necking does not occur, with the proviso that LoWoTo=LWT.
This r value varies depending in which direction of a material sheet surface the tensile axis is taken, and is called “in-plane anisotropy”.
This in-plane anisotropy is generally evaluated in terms of a Δr value of the following formula by examining the deformation characteristics in each direction by changing θ that is an angle between the tensile direction of the tensile test piece and the rolling direction of the material:Δr=(ro−2r45+r90)/2
where ro, r45 and r90 are r values at angles corresponding to 0°, 45° and 90° of the tensile direction with respect to the material rolling direction, respectively.
It is believed that the earing described above is closely related with in-plane anisotropy, and the smaller the Δr value, the smaller becomes in-plane anisotropy.
Japanese Unexamined Patent Publication (Kokai) No. 2002-60900 describes a steel sheet for deep drawn cans having excellent earing resistance that is manufactured by setting a sheet thickness of an Al killed steel containing B added thereto to 0.15 to 0.60 mm, a Δr value to a range of +0.15 to −0.08 and a heating rate at the time of re-crystallization annealing to 5° C./sec.
Japanese Unexamined Patent Publication (Kokai) No. 2001-303181 describes a manufacturing method of can steel sheets having less skin coarsening after processing and small anisotropy using the steps of conducting lubrication rolling to a finish sheet thickness of 1.5 mm or below, conducting hot rolling and coiling the sheet, then applying pickling, cold rolling and annealing, and further conducting secondary rolling to manufacture can steel sheets, wherein manufacturing conditions are determined so that a value of a relational formula determined by a C content of the steel sheet, a coiling temperature (CT), a cold rolling reduction ratio (CR), a secondary rolling reduction ratio (DR), a hot rolled sheet thickness (t) and a maximum frictional coefficient (μ) falls within a predetermined range.
Japanese Unexamined Patent Publication (Kokai) No. 01-184252 disclosed a steel sheet for DI cans containing C: 0.0040 to 0.0600% and having excellent stretch-flange formability.
However, this steel sheet is for DI cans and has a thickness of more than 0.2 mm, and therefore characteristic of the steel sheet is different from the steel sheet of the present invention having a thickness of 0.2 mm or below.
Possibility of manufacturing of the steel sheet of T4 to DR9 from the component within the same range is not disclosed.